It is known to use a perforate endless conveyor belt for the dewatering and thickening of sludge. Sludge to be dewatered is treated with a polymer to effect flocculation of the solid material in the sludge, which flocculation permits separation of the water from the sludge. The flocculated sludge and water are deposited on the upper surface of a perforate conveyor belt. As the flocculated sludge and water travel along the belt, water drains through perforations in the belt by the action of gravity to dewater and thicken the sludge. Flow breakers of the type shown in U. S. Pat. 3,984,329 to Wenzel et al, which patent is hereby incorporated herein by reference, may be interposed in the path of the sludge being dewatered to plow furrows in the sludge and enhance the drainage of water from the flocculated, colloidal sludge.
Copending U.S. patent application Ser. No. 703,594 filed Feb. 20, 1985 by Austin et al, which is a continuation of now-abandoned U.S. patent application Ser. No. 513,438 filed July 13, 1983, which applications are hereby incorporated herein by reference, disclose an improvement in dewatering devices of this general type. The improved dewatering device includes a ramp adjacent the output end of the perforate conveyor belt, which ramp causes the dewatered sludge to ascend the ramp. The ramp extends substantially across the width of the conveying surface and is set at a sufficient height and angle as to create an optimum backward rolling action of the sludge. This, in turn, imparts to the particles of the sludge a shearing action which further relieves trapped water from between the particles to enhance dewatering.
It has been discovered that the optimum angle of inclination for the ramp with respect to the conveyor is not the same for all sludges. Indeed, even at the same treatment plant, characteristics of the sludge may vary over the course of a day. Such characteristics tend to be particularly variable at different seasons.
Some waste activated sludges are characterized by a very delicate floc or colloidal structure created by the polymer, which floc can be broken up if the sludge is subjected to too much agitation or disturbance. Breaking up of the floc, in turn, causes clogging of the drainage passageways through the perforate belt. This, in turn, greatly diminishes or destroys the dewatering capabilities of the belt filter dewatering device. The performance can only be restored by interrupting the dewatering process and cleaning the clogged belt. For delicate sludges of this type, optimum dewatering is achieved with a relatively small angle of incline of the ramp with respect to the belt which, in turn, effects a less robust rolling back action of the sludge upon itself as the sludge tends to move up the ramp. This, in turn, results in a more gentle rolling and shearing action so as not to destroy the floc and so as to avoid clogging of the belt.
In other, less delicate, yet watery, sludges, a relatively low angle of incline will result in less than optimum production of solids by the dewatering device. Such sludges are better handled by a ramp with a steeper incline, which produces a more vigorous rolling back action.
Sludges which already have a low water content when they reach the ramp are better handled with a lower angle of incline and a less rolling back action.